Is Your Unsuspecting Bovine Protected From BVDV Infection?
by Dr. Lana Kaiser

In 1997 I wrote about BVD in the Voice, back then BVD (bovine viral diarrhea) was one of the most confusing and economically devastating diseases in the cattle business. Seven years later BVD is still a major problem. Infections are seen in all ages of cattle throughout the world and there is still a significant economic impact due to production and reproduction losses. If the disease has not really changed, why write about it again? Not only do cattle producers need  to have a basic understanding of the disease, but they also have few new things to think about. BVD, the disease of 1,000 faces, can masquerade as respiratory problems, abortion, diarrhea, decreased fertility, congenital defects in calves, hemorrhage and death. The terminology is confusing, and to add to the confusion, new vaccines are on the market claiming to provide fetal protection and contain both Type I and Type II BVD. There is a new test for persistently infected animals (do you remember the dreaded PI?) called skin biopsy immunohistochemistry – the ear notch test. The ear notch test is a great aid in detecting and ridding your operation of persistently infected cattle.

BVD is the original name for a gastroenteritis with severe diarrhea, first described in the 1940’s in small dairy herds in New York. At the time, both the causative organism and the magnitude of the disease were unknown, so bovine viral diarrhea (BVD) seemed to be a reasonable name. We now know the virus that causes BVD (termed BVDV, bovine viral diarrhea virus) is responsible for many different clinical diseases. BVDV, the cause of the disease in cattle, is an RNA virus losely related the virus that causes hog cholera and border disease in sheep. The virus is divided into several overlapping categories based on how the particular virus behaves in the laboratory, and having nothing to do with the virulence or behavior of the virus in cattle. The virus can be called Type I or Type II and cytopathic or non- cytopathic. You can have a Type I cytopathic virus, a Type I non- cytopathic virus, Type II cytopathic virus and a Type II non- ytopathic virus. There is really not one BVDV, but probably hundreds of slightly different viruses under the umbrella of BVDV.

The designation of the virus as Type I or Type II is based solely on its genetic sequence. The designation of the virus as cytopathic or non- cytopathic is based on how the virus behaves in cell culture; if it kills cells in cell culture it is called cytopathic, if it does not kill cells it is called non-cytopathic. A Type I virus can be cytopathic or non- cytopathic and a Type II virus can be cytopathic or non- cytopathic. This designation is not related to virulence, as both cytopathic and non-cytopathic strains can be very virulent or very mild and both Type I and Type II can cause mild or devastating disease.

The clinical conditions associated with this virus can be broadly divided into 3 categories: acute disease (sometimes referred to as primary infection), fetal infections and mucosal disease. Acute infections can be so subtle that you don’t notice them (subclinical) or so horrendous that the animal is dead before you know it. Let’s talk about them one by one. 

Acute BVDV Infection
Acute BVDV infection can be so subtle that you don’t realize the animal is sick; however, if the cow is pregnant and unvaccinated, the results can be devastating (see fetal infection). Bad things can happen right under your nose and you don’t even know it! Acute BVDV infection can present a sudden death or a viral respiratory disease, diarrhea, a severe bleeding disorder or a venereal disease. BVDV infection can also cause immunosuppression, the virus itself may not make the animal sick, but makes it more likely to fall prey to other diseases such as shipping fever or coccidiosis. You may think the animal died of pneumonia, but it was really BVDV that threw them over the edge. Most animals recover from acute infection. If they recover, they have antibodies against the virus said to be seropositive and have developed some immunity against the virus.

How does an animal get an acute infection? Just like you get the flu, you get exposed to someone carrying the bug. If the cattle are vaccinated, they have more resistance to the bug and may get the infection but do not show clinical signs of disease. Shows, fairs, auction yards, commingled cattle at sales, buying animals and not isolating them before you put them with your herd can all result in acute BVDV infection.

Fetal Infections
Fetal infections are a huge problem. The outcome of the fetal infection depends on the pregnancy the dam when she develops an acute BVDV infection. The most devastating result of fetal infection is the persistently infected calf. This dreaded creature is the result of maternal infection (a virus that crosses the placenta to the calf) during the first trimester. At this time the immune system is developing, and instead of recognizing BVDV as foreign (to be killed) it recognizes it as part of self. The calf is born with a high viral load (thinks the virus is part of self) and will shed lots of virus forever! This is the dreaded PI animal who can wreak havoc in your herd. PI animals are infected with non- cytopathic BVDV. Many PI calves die young, shortly after birth, at weaning or before their first birthday; however, some go merrily along, looking like any other calf, grow up, get bred (if heifers) and have babies. Calves born from PI cows are always PI. PI bull calves can go out into the world to breed cows. Imagine the mess it would be if you had a PI bull breeding your unvaccinated cows! You would see the many faces of BVDV infection right before your eyes. Even with a closed herd (we have a closed herd, we only buy a few bulls) bringing a PI bull into your herd can dramatically decrease your conception rate, number of calves born and number of calves weaned. The most important thing to remember about PI cattle is they are the Typhoid Mary’s of BVDV. They are the ones who spread the virus on the farm and bring it onto your farm – should you be so lucky to purchase one!!!

How do we get a PI animal? They are always born. The dam developed a BVDV infection while pregnant, either as the result of exposure to an animal with acute infection or a PI animal. Fetal infection can also result in early embryonic death, abortion, mummified fetus, congenital problems including cataracts and cerebellar hypoplasia and skeletal malformations. Dams infected late in pregnancy may actually have normal calves. However, recent research suggests even fetal infections late in pregnancy may result in the birth of calves that may look normal, but have subtle problems that may prevent them from being as productive.

Mucosal Disease
Although Mucosal disease is the classic syndrome often associated with BVDV infection, it is very rare. Mucosal disease occurs when a PI animal (infected with a non- cytopathic strain of BVDV) is superinfected with a similar strain of cytopathic BVDV. The virus causes destruction of intestinal tissue with diarrhea and death.

Stamping out BVDV on a farm is a 3-pronged approach – biosecurity, vaccination and testing.
What is new in the battle to stamp out BVDV?

Several companies have developed vaccines containing both BVDV Type I and BVDV Type II. This should provide broader protection. There are however, many different strains of both Type I and Type II, so vaccination does not guarantee your cattle will not develop infection or that the fetus is protected from infection. This means in addition to vaccination you should have a biosecurity and testing program in place. Before you give your pregnant cows modified live vaccine, READ the label directions. Although they say you can use these vaccines in pregnant cows, you can only use them in pregnant cows that have been vaccinated with the same product within the past 12 months!

The other thing new on the BVDV front is the ear notch. This is a great test for detecting PI animals. It is reasonably easy, economical, sensitive and specific, meaning if the test is positive the animal is a PI and if the test is negative the animal is not. The test identifies viral antigen in a skin sample. The easiest way to do the test is to ear notch (with pig ear notchers) calves (figure 1), although some people use a paper punch to get a circle of tissue.

There are currently three different types of tests used to identify the BVDV in the ear notch test. All depend on a tagged antibody attaching to the BVDV. The tagged antibody either shows up fluorescence (under black light) or as a specific color. Looking at a slide under a microscope of the fluorescent antibody test and identifying the bright green dots (figure 2) tells you that you have BVDV, likely a PI animal. The immunohistochemistry (IHC) technique shows the virus- ntibody  complex as red dots, usually around the hair follicle (figure 3). The final test is an ELISA, most of us are familiar with this type of test used on serum, but in this case the virus is eluted from the ear notch, and then the ELISA run in the usual fashion.

For FA, we put each ear notch in a baggie labeled with the animal identification, put them in the freezer slide and send them to the lab as a batch. DO NOT thaw the notches prior to taking them to the lab, freezing and thawing destroys the virus. The lab does five samples per slide, stains and reads them. This is an excellent way to find a PI animal in the herd. It can also be used on adult animals. Contact your veterinarian or laboratory to determine which test they use and how they want the samples handled.